These only occur in particular sorts of materials, such as semiconductors. An exciton is made up of an electron and a "hole". They have opposite charges and attract each other - they can be thought of as orbiting each other. Because of their mutual attraction, an exciton is energetically more favourable than the free components,

Since this is an electron orbiting a lack of an electron, it is possible for the electron to jump into the hole, causing the exciton to vanish. Because this recombination releases energy, this process usually emits light. However, the right conditions for both free electrons and holes to exist at the same time rarely occur; but we can make situations where they do. Excitons can interact with light to form exciton polaritons.